"This should be the last oil slick that will come to shore. It is stuck in one particular corner of the shore close to some rocks."

About 50,000 litres of crude oil poured into the sea on Saturday around 20km off the coast of Rayong, southeast of the capital, Bangkok. The company estimates that about 5,000 litres remain to be cleaned.

The oil reached Koh Samet, whose white sandy beaches are flocked by local and foreign tourists, owing to its proximity to Bangkok. The district is also full of several high-end and budget resorts.

Tourists on Koh Samet island were warned to stay away from the beach marred by inky globs as hundreds of workers in white jumpsuits laboured to scrape the sand clean and remove the oil from the water.

Wichit Chartpaisit, the governor of Rayong province, said that about 200 soldiers have been deployed to help clean up the spill and 300 more will provide back-up. He added that the task would take some time, but gave no details of how much.

Another subsidiary, PTT Exploration and Production Pcl, was involved in Australia's worst offshore drilling accident in 2009, when thousands of gallons of crude oil spewed into the sea after a damaged oil well blew up.

An Australian government inquiry blamed the spill on systemic shortcomings at the Thai oil giant.